Events refers both to a design pattern used for the asynchronous handling of various incidents which occur in the lifetime of a web page and to the naming, characterization, and use of a large number of incidents of different types.

The overview page provides an introduction to the design pattern and a summary of the types of incidents which are defined and reacted to by modern web browsers.

The custom events page describes how the event code design pattern can be used in custom code to define new event types emitted by user objects, register listener functions to handle those events, and trigger the events in user code.

The remaining pages describe how to use events of different kinds defined by web browsers. Unfortunately, these events have been defined piece by piece as web browsers have evolved so that there is no satisfying systematic characterization of the events built-in or defined by modern web browsers.

The window in which the browser is displayed can trigger events; for example, change size if the user maximizes the window or otherwise changes it.

The process loading of a web page can trigger events in response to the completion of different steps in the downloading, parsing, and rendering of the web page for display to the user.

The user interaction with the web page contents can trigger events. The events triggered by user interaction evolved during the early years of browser design and include a complicated system defining the sequence in which events will be called and the manner in which that sequence can be controlled. The different types of user interaction-driven events include:

The modification of the web page in structure or content might trigger some events, as explained in the mutation events page, but the use of these events has been deprecated in favour of the lighter Mutation Observer approach.

The media streams embedded in the HTML documents might trigger some events, as explained in the media events page.

The network requests made by a web page might trigger some events.

There are many other sources of events defined by web browsers for which pages are not yet available in this guide.

Note: This Event Developer Guide needs substantial work. The structure needs to be reorganized and the pages rewritten. Our hope is that everything you need to know about events will go under here.